The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services. Market demands for such spacecraft have imposed increasingly stringent requirements for payload flexibility and utilization efficiency. For example, there is an increased need for multi-port Amplifiers (MPAs) to provide more flexibility to move power from one beam to another, depending on the traffic demand.
An MPA is a circuit arrangement where an array of high power amplifiers, a N port input network, and a N port output network are configured such that the total output power provided by the array of amplifiers can be reapportioned at the N output ports of the output network, in relation to the signals at the N input ports of the input network. MPAs may be attractive for satellite payloads that utilize multiple beam antenna configurations. For example, MPAs may allow efficient and flexible sharing of total power from each power amplifier to be shared amongst the multiple beams, and permit adapting to the needs of variable traffic conditions or link conditions.
In the absence of an MPA scheme, one high power amplifier is associated with a respective antenna beam. In the absence of redundancy schemes, the failure of one high power amplifier may lead to the loss of the respective antenna beam. Furthermore, there is no way to redirect RF power from one high power amplifier to another antenna beam.
In contrast, in an MPA, the RF power at one of the beams in a multibeam configuration is a fraction of the total available RF power from the collection of HPAs. Failure of one high power amplifier reduces total RF output power, but does not cause total loss of a beam. Moreover, adjustment of the input signals permits increased flexibility in apportioning total available RF power amongst the multiple antenna beams.
An elemental implementation of an MPA includes two high power amplifiers sandwiched between two quadrature hybrid couplers, resulting in a configuration with two input ports and two output ports. When scattering parameters of the two amplifiers are identical, for a voltage applied at one of the input ports, all reflected voltages will sum to the termination at the second input port (referred to as the isolated input port). Moreover, the output power at one of the output ports will be twice that of a single amplifier, with no power lost at the second output port (referred to as the isolated output port).
When non-coherent inputs are present at each of the two input ports, based on the superposition principle, the signal on a particular input port will be output at its respective output port, and will not be output on the associated isolated output port.
By adjusting the amplitude ratio between the voltage inputs at the first and second input ports, the apportioning of total RF output power amongst the two output ports can be divided continuously between the opposite extreme ends.
A general topology of an MPA may include an array of high power amplifiers, an input multiport network, and an output multiport network. In certain embodiments, the input and output network can be in the form of Butler matrices. When the number of beams is less than an integer power of 2, the unused input/output ports can be terminated by matched loads.
For a theoretically ideal MPA implementation, a signal at one input port, after being split in the N port input network and amplified by the N HPAs, will be output at a single output port with no leakage of power to the associated N-1 isolated output ports. A real MPA, due to amplitude/phase imbalances in the input/output network, and gain/phase nonuniformity in the HPA array, will deviate from the ideal case. In other words, a signal at one input port will be output at more than one output port.
Thus, MPA performance is susceptible to degradation due to changes in gain and phase of the MPA components, which may vary significantly over the life of the satellite. In particular, components of a travelling wave tube amplifier (TWTA) or a solid-state power amplifier (SSPA) included in the MPA may exhibit gain and phase variation, particularly for high-frequency applications using the Ku, Ka and higher frequency bands. At Ku band, for example, relative phase variations between the set of TWTAs used in an MPA can be as large as 20 degrees over a 15 year satellite life, and gain variations can be of the order of 1 dB over 15 years.
In the absence of the presently disclosed techniques, calibration of these gain and phase variations over life and compensation for the variations are essential to the proper functioning of the MPAs. Otherwise, in many applications, these variations result in leakage of desired channel power into adjacent channels, which can manifest as coherent interference or multipath effects, as well as a reduction in power at the desired output ports. Many schemes for onboard and ground-based compensation have been proposed, including, for example, U.S. Pat. Nos. and 8,103,225, 8,463,204, 8,588,343, 8,737,528. These known techniques require the addition of amplitude and phase sensing components on the spacecraft, together with a means of adjusting amplitude and phase in the MPA, whether or not a ground station is involved in calculating the necessary adjustments. All of this leads to additional cost and complexity.
Thus, there is a desire to find techniques that permit usage of MPAs while avoiding the above-mentioned calibration and compensation schemes.